In recent years, personal computers have spread widely even among ordinary households, and it has become easy to use a videophone by means of moving picture transmission, to edit a shot movie image, and to do the like. The start of terrestrial digital broadcasting and the spread of DVD (digital versatile disk) recorders and hard disk recorders have further popularized digital moving images.
A digital moving picture generally has an enormous amount of digital information, and is therefore usually compressed. A moving image compression standard, MPEG-2 (ISO/IEC13818-2 International Standard MPEG-2 Video) is used for the above-mentioned terrestrial digital broadcasting and DVD recorders. A moving image compression standard, ITU-T H.263 is often used for videophones.
MPEG standards and many other moving image compression standards incorporate inter-frame compression techniques that exploit the temporal redundancy in successive image frames. Those techniques focus on the fact that successive frames are highly correlated with each other over such a short time as 30 frames per second. In concrete terms, one specific frame is encoded by using only information in the frame; and a frame following the frame is encoded by obtaining a difference frame that is a difference between the following frame and the preceding frame. A frame that is encoded by using only information in the frame is called an intra-frame encoded frame or an intraframe. An encoded following frame is called an inter-frame encoded frame or an interframe, since it is encoded by using a preceding frame.
At decoding, an intraframe is first subjected to intra-frame decoding, and then following interframes are decoded in order. An interframe is decoded by decoding a difference frame first and then adding the decoded difference frame to the preceding frame.
In actual inter-frame compression encoding, a frame is divided into several blocks, and intra-frame encoding or inter-frame encoding is chosen block by block. A block subjected to intra-frame encoding is called an intrablock, and a block subjected to inter-frame encoding is called an interblock. An intraframe is a frame consisting only of intrablocks, and an interframe is a frame including an interblock.
Encoding an interblock not only simply determines a difference between blocks, but also employs a motion vector. This allows to determine a difference between a block to be encoded and a block with which the block to be encoded is most highly correlated, and thereby improves compression ratio.
An intraframe can be decoded immediately. An interframe includes an interblock, so it can be decoded only if it is added with a block in the already-decoded preceding frame. An interframe cannot be decoded until the preceding frame is decoded. That is, an interframe cannot be decoded until preceding frames have been decoded in order from the nearest preceding intraframe. This is a problem common to inter-frame compression techniques that exploit the temporal redundancy. As a result, an image recorded by a DVD recorder cannot be started from an arbitrary point in its random-access playback, and a playback starting point is limited to where an intraframe was inserted. This problem can seemingly be avoided by frequently inserting intraframes. However, since the compression ratio of an intraframe is low, insertion of many intraframes would significantly make the compression ratio of a moving image worse.
One example of the inconvenience caused by the fact that an inter-frame compressed moving image cannot be started to decode with an interframe is participation in a multipoint videoconference. There are two methods to implement a multipoint videoconference. One method is that moving images shot by each terminal are gathered to a multipoint conference apparatus called an MCU, and that the MCU combines all the moving images and then sends them back to each terminal. The other method is that a terminal on the transmitting side makes a multi-address transmission of a moving image shot by the terminal to all terminals, and that a terminal on the receiving side combines transmitted moving images. In either method, if an inter-frame compression technique is used for moving image compression, a terminal that newly participates during a conference cannot start to decode with an interframe, and therefore the terminal starts to play after the next intraframe arrives. As a result, a condition occurs in which no image is displayed for some time despite the participation in the conference.
Another example of the inconvenience caused by the fact that an inter-frame compressed moving image cannot be started to decode with an interframe is the start of play or channel switching in a digital moving image broadcasting service. Terrestrial digital broadcasting, mobile broadcasting using a satellite, and the like have been started as broadcasting services. A decoder used in a digital television or other terminals, when broadcasting reception is started or when a channel is switched, starts to play the receiving channel after a next intraframe arrives.
Japanese Patent Laid-Open Publication No. Heisei 7-298231 discloses a technique for decoding and displaying an image immediately after participation of a new terminal in a multipoint videoconference. According to Japanese Patent Laid-Open Application No. Heisei 7-298231, a participant terminal notifies, of its participation, an MCU in a first method of implementing a videoconference, or all terminals which have already participated in a conference in a second method of implementing a videoconference. In response to the notification, the MCU or all the terminals participating in the conference subject moving images shot by the terminals to intra-frame encoding and send intraframes for a certain period of time. This allows the new participant terminal to immediately decode and display frames.